Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al, as well as structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversions, which can take place on outside surfaces as well as on internal surfaces within the pore.
Applicants have synthesized a new family of materials designated UZM-8. The UZM-8 compositions are aluminosilicates having Si/Al molar ratio from about 6.5 to about 35. The UZM-8 compositions show unique x-ray diffraction patterns compared to other known zeolites. These UZM-8 compositions are prepared from aqueous reaction mixtures containing either organoammonium compounds or a mixture of organoammonium compounds and alkali and/or alkaline earth compounds. The organoammonium compounds used to make UZM-8 are non-cyclic nor contain cyclic substituents and are generally quite simple. Preferred examples of organoammonium compounds used to make UZM-8 include the diethyldimethylammonium (DEDMA), ethyltrimethylammonium (ETMA) or hexamethonium (HM) cations.
Although UZM-8 compositions have some similarities to a layered material identified as MCM-56, there are sufficient differences that UZM-8 compositions are structurally different from MCM-56 materials and thus are unique new zeolites structures. The preparation of MCM-56 is disclosed in U.S. Pat. No. 5,362,697 where it is stated that MCM-56 is prepared from a reaction mixture containing a combination of alkali metals and hexamethylene imine (HMI) as directing agents and requires that the silica source be a predominately solid silica source comprising at least 30 wt. % SiO2. It is further stated in the '697 patent that the reaction must be stopped and quenched at a time before significant amounts of MCM-49 form in the reaction mixture. The synthesis of MCM-49 is disclosed in U.S. Pat. No. 5,236,575 and again involves a combination of alkali metals and HMI structure directing agents plus a predominately solid silica source comprising at least 30 wt. % SiO2. Upon calcination the MCM-49 composition is not readily distinguishable from calcined MCM-22 which has the MWW framework topology. It is further stated in J. Phys. Chem., 1996, 100, p.3788–3798, that in the as-synthesized form MCM-49 has essentially the MWW topology. Thus, MCM-56 is an intermediate structure in the formation of MCM-49 which in the calcined form is virtually the same as MCM-22 both of which have the MWW structure. The '697 patent further describes the MCM-56 as a layered structure in both its as-synthesized and calcined forms based on the claimed swellability of the material.
In contrast to MCM-56, UZM-8 is not an intermediate in the formation of MCM-49. Additionally, the UZM-8 materials can be synthesized from an alkali free reaction mixture using an organoammonium cation such as DEDMA cation that offers great stability and robustness without the formation of MCM-49 or other impurities. However, in the HMI/Na system, varying the relative amount of amine structure directing agent to alkali metal and/or alkaline earth metal compound can yield either the MCM-56/MCM-49 system with higher relative alkali content or a precursor to MCM-22 with lower relative alkali content. Reaction conditions, mainly temperature and time, are used to distinguish MCM-56 and MCM-49 in the higher alkali content system that is difficult to control, leading to the requirement for quenching the MCM-56 reaction mixture before significant amounts of MCM-49 form. Finally, UZM-8 is a layered material in that the as-synthesized form is swellable and has a x-ray diffraction pattern that is distinguishable from MCM-56.
The UZM-8 compositions have also been modified by using one or more techniques selected from acid extraction, calcination, steaming and ammonium hexafluorosilicate treatment, applicants have been able to control the aluminum content of the UZM-8 zeolites to nearly all silica while maintaining their structure and porosity. Dealumination strategies are known in the art and are given by Breck (see D. W. Breck, Zeolite Molecular Sieves, Wiley and Sons, New York, (1974), p. 441) and Skeels and Breck (see U.S. Pat. No. 4,610,856). The result is a modified UZM-8 (UZM-8HS) material containing less aluminum than the starting UZM-8 composition. Control of the Al content in the zeolite allows one to tune the properties associated with the Al, such as ion-exchange capacity and acidity thereby providing improved catalysts and/or adsorbents. These modified compositions have been designated UZM-8HS.